This invention relates in general to a functional integration of multiple components for a fuel cell power plant, and deals more particularly with the functional integration of multiple components of a fuel cell power plant whereby the strong characteristics of each component may be utilized to compensate for the weak characteristics of the other components.
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a fuel being provided to an anode electrode and an oxidant being provided to a cathode electrode, generating an external current flow there-between. Such fuel cell assemblies are very useful due to their high efficiency, as compared to internal combustion fuel systems and the like, and may be applied in many fields. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products, typically water, which are produced during their operation. Owing to these characteristics, amongst others, fuel cell assemblies are particularly applicable in those fields requiring highly reliable, stand-alone power generation, such as is required in space vehicles and mobile units including generators and motorized vehicles.
Electrochemical fuel cell assemblies typically employ a hydrogen rich gas stream as a fuel and an oxygen rich gas stream as an oxidant where the reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the anode and cathode electrode substrates formed of porous, electrically conductive sheet materialxe2x80x94typically, carbon fiber paper. One particular type of ion exchange membrane is known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION(trademark) and well known in the art. Catalyst layers are formed between the PEM and each electrode substrate to promote the desired electrochemical reaction. The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly.
Other commonly known electrolytes used in fuel cell assemblies include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix. It has, however, been found that PEM fuel cell assemblies have substantial advantages over fuel cells with liquid or alkaline electrolytes due to the superior performance of the PEM in providing a barrier between the circulating fuel and oxidant, while also being more tolerant to pressure differentials than a liquid electrolyte that is held by capillary forces within a porous matrix. Moreover, a PEM electrolyte is fixed and will not leach from the fuel cell assembly and retains a relatively stable capacity for water retention.
In the typical operation of a PEM fuel cell assembly, a hydrogen rich fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the PEM to the cathode electrode while the electrons flow through an external circuit to the cathode electrode. At the cathode electrode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode electrode at the catalyst layer to form the by-product water. Not only does the PEM facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen rich fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers are represented by the equations:
H2xe2x86x922H++2exe2x80x83xe2x80x83Anode reaction
xc2xdO2+2H++2exe2x86x92H2Oxe2x80x83xe2x80x83Cathode reaction
Conventional PEM fuels cells have the ion exchange membrane positioned between two permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite-polymer composite, or the like. The plates act as a structural support for the two porous, electrically conductive electrode substrates, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode electrode and cathode electrode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
Moreover, the plates may have formed therein reactant feed manifolds which are utilized for supplying the fuel to the anode flow channels or, alternatively, the oxidant to the cathode flow channels. They may also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. The construction and operation of a typical PEM fuel cell are well known and are described in detail in commonly owned U.S. Pat. No. 5,853,909, issued to Reiser, and incorporated herein by reference in its entirety. Alternatively, the manifolds may be external to the fuel cell itself, as shown in commonly owned U.S. Pat. No. 3,994,748, issued to Kunz et al., and incorporated herein by reference in its entirety.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing a hydrogen rich gas produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline or the like, into a hydrogen rich stream. This process requires that the hydrogen produced must be efficiently converted to be as pure as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. This conversion of hydrocarbons is generally accomplished through the use of a steam reformer or an autothermal reformer. Reformed hydrocarbon fuels frequently contain quantities of ammonia, NH3, as well as significant quantities of carbon dioxide, CO2. These gases tend to dissolve and dissociate into the water which is provided to, and created within, the fuel cell assembly. The resultant contaminated water supply may cause the conductivity of the water to increase to a point where shunt current corrosion occurs in the coolant channels and manifold leading to degradation of fuel cell materials, as well as reducing the electrical conductivity of the PEM and thereby reducing the efficiency of the fuel cell assembly as a whole.
As disclosed above, the anode and cathode plates may be part of a coolant loop which provides coolant channels for the circulation of a water coolant, as well as for the wicking and carrying away of excessive water produced as a by-product of the fuel cell assembly operation. The water which is collected and circulated through a fuel cell assembly is therefore susceptible to contamination and may damage and impair the operation of the fuel cell assembly.
It is therefore necessary to provide a system which protects the fuel cell assembly from water contamination. One such system is described in commonly owned U.S. Pat. No. 4,344,850, issued to Grasso, and incorporated herein by reference in its entirety. Grasso""s system for treating the coolant supply of a fuel cell assembly, as illustrated in FIG. 1 of U.S. Pat. No. 4,344,850, utilizes a separate filter and demineralizer for purifying a portion of the coolant supplied to the fuel cell assembly. A separate degasifier is also utilized to process the condensed water obtained from a humidified cathode exit stream. As discussed in Grasso, the heat exchange occurring between the coolant stream and the body of the fuel cell assembly is accomplished according to commonly assigned U.S. Pat. No. 4,233,369, issued to Breault et al., incorporated herein by reference in its entirety.
It is important to note that Grasso""s coolant system does not provide for the cleansing of the coolant stream as a whole. This is due to the fact that the coolant conduits in Grasso, being fashioned from copper or the like, are not in diffusable communication with the body of the fuel cell assembly and as such, the coolant stream does not receive contamination from, inter alia, the CO2 or NH3 present in the reformed fuel stream. The burden of cleansing the coolant stream in Grasso is therefore born solely by the filter and demineralizer and results in greater wear on these components and hence greater repairs and replacements. Grasso also utilizes two distinct coolant pumps for circulating the coolant.
Another coolant treatment system has been disclosed in commonly assigned co-pending U.S. patent application Ser. No. 09/295,732, entitled xe2x80x9cWater Treatment System for a Fuel Cell Assemblyxe2x80x9d, herein incorporated by reference in its entirety. U.S. patent application Ser. No. 09/295,732 utilizes an unique arrangement of separate demineralizers and degasifiers to cleanse the entire circulating coolant stream while providing for the humidification of an inputted oxidant stream.
Complicating the objective of providing a coolant treatment system is the cumulative effect that the multitude of components in a fuel cell power plant have on the overall weight, volume, and complexity of the system.
In practical applications, a plurality of planar fuel cell assemblies are typically arranged in series, commonly referred to as a cell stack assembly. The cell stack assembly may be surrounded by an electrically insulating housing that defines the various manifolds necessary for directing the flow of a hydrogen rich fuel and an oxygen rich oxidant to the individual fuel cell assemblies, as well as a coolant stream, in a manner well known in the art. The cell stack assembly, including any associated components such as a degasifier, a demineralizer, a steam reformer, a heat exchanger and the like may, as a whole, be referred to as a fuel cell power plant.
As will be appreciated by one so skilled in the art, integrating these differing components into a cohesive fuel cell power plant operating within specific design parameters results in a complex and oftentimes cumbersome structure.
Specifically, in the operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode, including water resulting from proton drag through the PEM electrolyte, and rates at which water is removed from the cathode or supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain an optimal water balance as the electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out, thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM thus decreasing cell performance.
Given that fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks, buses, etc., maintaining a water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize fuel cell power plant weight and space requirements, the plant must be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset water losses from gaseous streams of reactant fluids passing through the plant. For example, any water exiting the plant through a cathode exhaust stream of gaseous oxidant or through an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode and retained within the plant.
An additional requirement for maintaining water self-sufficiency in fuel cell power plants is associated with components necessary to process hydrocarbon fuels, such as methane, natural gas, gasoline, methanol, diesel fuel, etc., into an appropriate reducing fluid that provides a hydrogen rich fluid to the anode electrode. Such fuel processing components of a fuel cell power plant typically include a boiler that generates steam; a steam duct into which the hydrocarbon fuel is injected; and an autothermal reformer that receives the steam and fuel mixture along with a small amount of a process oxidant such as air and transforms the mixture into a hydrogen-rich reducing fluid appropriate for delivery to the anode electrode of the fuel cell. The fuel processing components or system water and energy requirements are part of an overall water balance and energy requirement of the fuel cell power plant. Water made into steam in the boiler must be replaced by water recovered from the plant such as by condensing heat exchangers in the cathode exhaust stream and associated piping.
A common approach to enhancing water recovery and retention is use of condensing heat exchangers in exhaust streams of the power plant wherein the exhaust streams are cooled to a temperature at or below their dew points to condense liquid water from the exhaust streams so that the liquid may be returned to the power plant. An example of a PEM fuel cell power plant using a condensing heat exchanger is shown in U.S. Pat. No. 5,573,866 that issued on Nov. 12, 1996, to Van Dine et al., and is assigned to the assignee of the present invention, and which patent is hereby incorporated herein by reference in its entirety. Many other fuel cell power plants that use one or more condensing heat exchangers are well-known in the art, and they typically use ambient air streams as a cooling fluid passing through the heat exchanger to cool the plant exhaust streams. In Van Dine et al., the heat exchanger is used to cool a cathode exhaust stream, which upon leaving a cathode chamber includes evaporated product water and some portion of methanol, the reducing fluid, that has passed through the PEM. The condensing heat exchanger passes the cathode exhaust stream in heat exchange relationship with a stream of cooling ambient air, and then directs condensed methanol and water indirectly through a piping system back to an anode side of the cell.
While condensing heat exchangers have enhanced the water recovery and energy efficiency of fuel cell power plants, the heat exchangers encounter decreasing water recovery efficiency as ambient temperatures increase. Where the power plant is to power a transportation vehicle such as an automobile, the plant will be exposed to an extremely wide range of ambient temperatures. For example, where an ambient air coolant stream passes through a heat exchanger, performance of the exchanger will vary as a direct function of the temperature of the ambient air because decreasing amounts of liquid condense out of power plant exhaust streams as the ambient air temperature increases.
An additional requirement of using such condensing heat exchangers in fuel cell power plants powering transportation vehicles is related to operation of the vehicles in temperatures below the freezing temperature of water. Because water from such exchangers is often reintroduced into the PEM fuel cells of the plant, the water may not be mixed with conventional antifreeze to lower its freezing temperature. Propylene glycol and similar antifreezes would be adsorbed by the catalysts in the cells decreasing cell efficiency, as is well known.
Accordingly, known fuel cell power plants that employ ambient air as the cathode oxidant and/or that use condensing heat exchangers are incapable of efficiently maintaining a self-sufficient water balance when operating at high ambient temperatures because of their above described characteristics. It is therefore highly desirable to produce a fuel cell power plant that can achieve a self-sufficient water balance without a condensing heat exchanger while minimizing plant operating energy requirements.
Commonly assigned U.S. patent application Ser. No. 09/395,704, entitled xe2x80x9cFine Pore Enthalpy Exchange Barrier For A Fuel Cell Power Plantxe2x80x9d, filed Sep. 14, 1999, herein incorporated by reference in its entirety, discloses one such method of integrating a fuel cell power plant heat exchange mechanism with a water balancing process by utilizing an enthalpy exchange barrier. By ensuring that an inlet and outlet oxidant stream are placed in fluid communication with one another on opposing sides of an enthalpy exchange barrier, the fuel cell power plant ensures that the inlet oxidant stream is sufficiently humidified while simultaneously decreasing the thermal and water content of the outlet oxidant stream. Commonly owned U.S. Pat. No. 6,007,931, entitled a xe2x80x9cMass and Heat Recovery System for a Fuel Cell Power Plantxe2x80x9d, discloses yet another method of integrating a fuel cell power plant heat exchange mechanism with a water balancing process and is herein incorporated by reference in its entirety.
With the forgoing problems and concerns in mind, the present invention seeks to provide a fuel cell power plant in which a multitude of separate components are integrated in order to allow the fuel cell power plant to operate at peak efficiency and to minimize the weight, volume and complexity of the power plant.
It is an object of the present invention to provide a functional integration of multiple components of a fuel cell power plant.
It is another object of the present invention to integrate the functions of multiple components of a fuel cell power plant so that the strengths of one component act to mitigate the weaknesses of another.
It is another object of the present invention to provide a functional integration of multiple components of a fuel cell power plant, which also humidifies the oxidant stream provided to the cathode of a fuel cell assembly.
It is another object of the present invention to reduce the possibility of contaminating gas build-up within the fuel cell power plant.
It is another object of the present invention to provide functional integration of multiple components to reduce the weight and volume of a fuel cell power plant.
According to one embodiment of the present invention, a fuel cell power plant having a plurality of functionally integrated components includes a fuel cell assembly provided with a fuel stream, an oxidant stream and a coolant stream. The fuel cell power plant functionally integrates a mass and heat recovery device for promoting a transfer of thermal energy and moisture between a first gaseous stream and a second gaseous stream, and a burner for processing the fuel exhausted from the fuel cell assembly during operation thereof. A housing chamber is utilized in which the oxidant stream exhausted from the fuel cell assembly merges with a burner gaseous stream exhausted from the burner. The resultant gaseous flow from the common chamber is directed back to the mass and heat recovery device as the first gaseous stream.
These and other objectives of the present invention, and their preferred embodiments, shall become clear by consideration of the specification, claims and drawings taken as a whole.